/*

Copyright (c) 2019, NVIDIA Corporation

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

*/

/*

This file introduces std::atomic_wait, atomic_notify_one, atomic_notify_all.

It has these strategies implemented:
 * Contention table. Used to optimize futex notify, or to hold CVs. Disable with __NO_TABLE.
 * Futex. Supported on Linux and Windows. For performance requires a table on Linux. Disable with __NO_FUTEX.
 * Condition variables. Supported on Linux and Mac. Requires table to function. Disable with __NO_CONDVAR.
 * Timed back-off. Supported on everything. Disable with __NO_SLEEP.
 * Spinlock. Supported on everything. Force with __NO_IDENT. Note: performance is too terrible to use.

You can also compare to pure spinning at algorithm level with __NO_WAIT.

The strategy is chosen this way, by platform:
 * Linux: default to futex (with table), fallback to futex (no table) -> CVs -> timed backoff -> spin.
 * Mac: default to CVs (table), fallback to timed backoff -> spin.
 * Windows: default to futex (no table), fallback to timed backoff -> spin.
 * CUDA: default to timed backoff, fallback to spin. (This is not all checked in in this tree.)
 * Unidentified platform: default to spin.

*/

//#define __NO_TABLE
//#define __NO_FUTEX
//#define __NO_CONDVAR
//#define __NO_SLEEP
//#define __NO_IDENT

// To benchmark against spinning
//#define __NO_WAIT

#ifndef __ATOMIC_WAIT_INCLUDED
#define __ATOMIC_WAIT_INCLUDED
#define __NO_SPIN

#include <cstdint>
#include <climits>
#include <cassert>
#include <type_traits>

#if defined(__NO_IDENT)

    #include <thread>
    #include <chrono>

    #define __ABI
    #define __YIELD() std::this_thread::yield()
    #define __SLEEP(x) std::this_thread::sleep_for(std::chrono::microseconds(x))
    #define __YIELD_PROCESSOR()

#else

#if defined(__CUSTD__)
    #define __NO_FUTEX
    #define __NO_CONDVAR
    #ifndef __CUDACC__
        #define __host__ 
        #define __device__
    #endif
    #define __ABI __host__ __device__
#else
    #define __ABI
#endif

#if defined(__APPLE__) || defined(__linux__)

    #include <unistd.h>
    #include <sched.h>
    #define __YIELD() sched_yield()
    #define __SLEEP(x) usleep(x)

    #if defined(__aarch64__)
        #  define __YIELD_PROCESSOR() asm volatile ("yield" ::: "memory")
    #elif defined(__x86_64__)
        # define __YIELD_PROCESSOR() asm volatile ("pause" ::: "memory")
    #elif defined (__powerpc__)
        # define __YIELD_PROCESSOR() asm volatile ("or 27,27,27" ::: "memory")
    #endif
#endif

#if defined(__linux__) && !defined(__NO_FUTEX)

    #if !defined(__NO_TABLE)
        #define __TABLE
    #endif

    #include <time.h>
    #include <unistd.h>
    #include <linux/futex.h>
    #include <sys/syscall.h>
    
    #define __FUTEX
    #define __FUTEX_TIMED
    #define __type_used_directly(_T) (std::is_same<typename std::remove_const< \
            typename std::remove_volatile<_Tp>::type>::type, __futex_preferred_t>::value)
    using __futex_preferred_t = std::int32_t;
    template <class _Tp, typename std::enable_if<__type_used_directly(_Tp), int>::type = 1>
    void __do_direct_wait(_Tp const* ptr, _Tp val, void const* timeout) {
        syscall(SYS_futex, ptr, FUTEX_WAIT_PRIVATE, val, timeout, 0, 0);
    }
    template <class _Tp, typename std::enable_if<__type_used_directly(_Tp), int>::type = 1>
    void __do_direct_wake(_Tp const* ptr, bool all) {
        syscall(SYS_futex, ptr, FUTEX_WAKE_PRIVATE, all ? INT_MAX : 1, 0, 0, 0);
    }

#elif defined(_WIN32) && !defined(__CUSTD__)

    #define __NO_CONDVAR
    #define __NO_TABLE

    #include <Windows.h>
    #define __YIELD() Sleep(0)
    #define __SLEEP(x) Sleep(x)
    #define __YIELD_PROCESSOR() YieldProcessor()

    #include <intrin.h>
    template <class _Tp>
    auto __atomic_load_n(_Tp const* a, int) -> typename std::remove_reference<decltype(*a)>::type {
        auto const t = *a;
        _ReadWriteBarrier();
        return t;
    }
    #define __builtin_expect(e, v) (e)

    #if defined(_WIN32_WINNT) && (_WIN32_WINNT >= _WIN32_WINNT_WIN8) && !defined(__NO_FUTEX)

        #define __FUTEX
        #define __type_used_directly(_T) (sizeof(_T) <= 8)
        using __futex_preferred_t = std::int64_t;
        template <class _Tp, typename std::enable_if<__type_used_directly(_Tp), int>::type = 1>
        void __do_direct_wait(_Tp const* ptr, _Tp val, void const*) {
            WaitOnAddress((PVOID)ptr, (PVOID)&val, sizeof(_Tp), INFINITE);
        }
        template <class _Tp, typename std::enable_if<__type_used_directly(_Tp), int>::type = 1>
        void __do_direct_wake(_Tp const* ptr, bool all) {
            if (all)
                WakeByAddressAll((PVOID)ptr);
            else
                WakeByAddressSingle((PVOID)ptr);
        }

    #endif
#endif // _WIN32

#if !defined(__FUTEX) && !defined(__NO_CONDVAR)

    #if defined(__NO_TABLE)
        #warning "Condvars always generate a table (ignoring __NO_TABLE)."
    #endif
    #include <pthread.h>
    #define __CONDVAR
    #define __TABLE
#endif

#endif // __NO_IDENT

#ifdef __TABLE
    struct alignas(64) contended_t {
    #if defined(__FUTEX)
        int                     waiters = 0;
        __futex_preferred_t     version = 0;
    #elif defined(__CONDVAR)
        int                     credit = 0;
        pthread_mutex_t         mutex = PTHREAD_MUTEX_INITIALIZER;
        pthread_cond_t          condvar = PTHREAD_COND_INITIALIZER;
    #else
        #error ""
    #endif
    };
    contended_t * __contention(volatile void const * p);
#else
    template <class _Tp>
    __ABI void __cxx_atomic_try_wait_slow_fallback(_Tp const* ptr, _Tp val, int order) {
    #ifndef __NO_SLEEP
        long history = 10;
        do {
            __SLEEP(history >> 2);
            history += history >> 2;
            if (history > (1 << 10))
                history = 1 << 10;
        } while (__atomic_load_n(ptr, order) == val);
    #else
        __YIELD();
    #endif
    }
#endif // __TABLE

#if defined(__CONDVAR)

    template <class _Tp>
    void __cxx_atomic_notify_all(volatile _Tp const* ptr) {
        auto * const c = __contention(ptr);
        __atomic_thread_fence(__ATOMIC_SEQ_CST);
        if(__builtin_expect(0 == __atomic_load_n(&c->credit, __ATOMIC_RELAXED), 1))
            return;
        if(0 != __atomic_exchange_n(&c->credit, 0, __ATOMIC_RELAXED)) {
            pthread_mutex_lock(&c->mutex);
            pthread_mutex_unlock(&c->mutex);
            pthread_cond_broadcast(&c->condvar);
        }
    }
    template <class _Tp>
    void __cxx_atomic_notify_one(volatile _Tp const* ptr) {
        __cxx_atomic_notify_all(ptr);
    }
    template <class _Tp>
    void __cxx_atomic_try_wait_slow(volatile _Tp const* ptr, _Tp const val, int order) {
        auto * const c = __contention(ptr);
        pthread_mutex_lock(&c->mutex);
        __atomic_store_n(&c->credit, 1, __ATOMIC_RELAXED);
        __atomic_thread_fence(__ATOMIC_SEQ_CST);
        if (val == __atomic_load_n(ptr, order))
            pthread_cond_wait(&c->condvar, &c->mutex);
        pthread_mutex_unlock(&c->mutex);
    }

#elif defined(__FUTEX)

        template <class _Tp, typename std::enable_if<!__type_used_directly(_Tp), int>::type = 1>
        void __cxx_atomic_notify_all(_Tp const* ptr) {
    #if defined(__TABLE)
            auto * const c = __contention(ptr);
            __atomic_fetch_add(&c->version, 1, __ATOMIC_RELAXED);
            __atomic_thread_fence(__ATOMIC_SEQ_CST);
            if (0 != __atomic_exchange_n(&c->waiters, 0, __ATOMIC_RELAXED))
                __do_direct_wake(&c->version, true);
    #endif
        }
        template <class _Tp, typename std::enable_if<!__type_used_directly(_Tp), int>::type = 1>
        void __cxx_atomic_notify_one(_Tp const* ptr) {
            __cxx_atomic_notify_all(ptr);
        }
        template <class _Tp, typename std::enable_if<!__type_used_directly(_Tp), int>::type = 1>
        void __cxx_atomic_try_wait_slow(_Tp const* ptr, _Tp const val, int order) {
    #if defined(__TABLE)
            auto * const c = __contention(ptr);
            __atomic_store_n(&c->waiters, 1, __ATOMIC_RELAXED);
            __atomic_thread_fence(__ATOMIC_SEQ_CST);
            auto const version = __atomic_load_n(&c->version, __ATOMIC_RELAXED);
            if (__builtin_expect(val != __atomic_load_n(ptr, order), 1))
                return;
        #ifdef __FUTEX_TIMED
            constexpr timespec timeout = { 2, 0 }; // Hedge on rare 'int version' aliasing.
            __do_direct_wait(&c->version, version, &timeout);
        #else
            __do_direct_wait(&c->version, version, nullptr);
        #endif
    #else
        __cxx_atomic_try_wait_slow_fallback(ptr, val, order);
    #endif // __TABLE
        }

    template <class _Tp, typename std::enable_if<__type_used_directly(_Tp), int>::type = 1>
    void __cxx_atomic_try_wait_slow(_Tp const* ptr, _Tp val, int order) {
    #ifdef __TABLE
        auto * const c = __contention(ptr);
        __atomic_fetch_add(&c->waiters, 1, __ATOMIC_RELAXED);
        __atomic_thread_fence(__ATOMIC_SEQ_CST);
    #endif
        __do_direct_wait(ptr, val, nullptr);
    #ifdef __TABLE
        __atomic_fetch_sub(&c->waiters, 1, __ATOMIC_RELAXED);
    #endif
    }
    template <class _Tp, typename std::enable_if<__type_used_directly(_Tp), int>::type = 1>
    void __cxx_atomic_notify_all(_Tp const* ptr) {
    #ifdef __TABLE
        auto * const c = __contention(ptr);
        __atomic_thread_fence(__ATOMIC_SEQ_CST);
        if (0 != __atomic_load_n(&c->waiters, __ATOMIC_RELAXED))
    #endif
            __do_direct_wake(ptr, true);
    }
    template <class _Tp, typename std::enable_if<__type_used_directly(_Tp), int>::type = 1>
    void __cxx_atomic_notify_one(_Tp const* ptr) {
    #ifdef __TABLE
        auto * const c = __contention(ptr);
        __atomic_thread_fence(__ATOMIC_SEQ_CST);
        if (0 != __atomic_load_n(&c->waiters, __ATOMIC_RELAXED))
    #endif
            __do_direct_wake(ptr, false);
    }

#else // __FUTEX || __CONDVAR

    template <class _Tp>
    __ABI void __cxx_atomic_try_wait_slow(_Tp const* ptr, _Tp val, int order) {
        __cxx_atomic_try_wait_slow_fallback(ptr, val, order);
    }
    template <class _Tp>
    __ABI void __cxx_atomic_notify_one(_Tp const* ptr) { }
    template <class _Tp>
    __ABI void __cxx_atomic_notify_all(_Tp const* ptr) { }

#endif // __FUTEX || __CONDVAR

template <class _Tp>
__ABI void __cxx_atomic_wait(_Tp const* ptr, _Tp const val, int order) {
#ifndef __NO_SPIN
    if(__builtin_expect(__atomic_load_n(ptr, order) != val,1))
        return;
    for(int i = 0; i < 16; ++i) {
        if(__atomic_load_n(ptr, order) != val)
            return;
        if(i < 12)
            __YIELD_PROCESSOR();
        else
            __YIELD();
    }
#endif
    while(val == __atomic_load_n(ptr, order))
#ifndef __NO_WAIT
        __cxx_atomic_try_wait_slow(ptr, val, order)
#endif
        ;
}

#include <atomic>

namespace cpp20 {

    template <class _Tp, class _Tv>
    __ABI void atomic_wait_explicit(std::atomic<_Tp> const* a, _Tv val, std::memory_order order) {
        __cxx_atomic_wait((const _Tp*)a, (_Tp)val, (int)order);
    }
    template <class _Tp, class _Tv>
    __ABI void atomic_wait(std::atomic<_Tp> const* a, _Tv val) {
        atomic_wait_explicit(a, val, std::memory_order_seq_cst);
    }
    template <class _Tp>
    __ABI void atomic_notify_one(std::atomic<_Tp> const* a) {
        __cxx_atomic_notify_one((const _Tp*)a);
    }
    template <class _Tp>
    __ABI void atomic_notify_all(std::atomic<_Tp> const* a) {
        __cxx_atomic_notify_all((const _Tp*)a);
    }
}

#undef __NO_SPIN
#endif //__ATOMIC_WAIT_INCLUDED
